1. Field of the Invention
The present invention relates to optical systems capable of spectral detection of light in a wide wavelength range from deep ultraviolet to near infrared. The present invention also relates to inspection apparatuses for inspecting, together with the optical systems, a test object having minute repeating pattern structures of several tens of nanometers in size formed on its substrate, namely patterned media such as semiconductor devices and next-generation hard disk media, in order to optically detect the shape of the pattern structures and whether the pattern structures are properly formed.
2. Description of the Related Art
There has been an ever-growing trend over the years to increase the storage capacity of hard disk drives more. However, conventional substrate disks with a magnetic film formed thereon, or equivalently, so-called continuous media, have a storage density, at its greatest, of approximately 1 Tbit/in2, which is deemed to be their limit. Instead of the continuous media, planned to be introduced are patterned media as a technique for achieving higher storage densities than 1 Tbit/in2.
Patterned media are classified into two types: discrete track media, as shown in FIG. 1B, having recording tracks 0102 with grooves formed therebetween on a disk 0101 shown in FIG. 1A; and bit-patterned media, as shown in FIG. 1C, having isolated islands 0103 each corresponding to a unit of recording (a bit). Unlike conventional continuous media, both patterned media are characterized in that the patterns are formed on a disk medium (substrate) so as to arrange the identically-shaped tracks or islands (pattern structures) at a pitch of tens of nanometers.
The patterned media require an additional new manufacturing process for patterning, which arouses a concern for defects caused by the process. FIG. 2, which is a schematic diagram of the cross sectional profile of the pattern, shows proper pattern structures 0201 and possible defects, for example, deformed pattern structures 0202 and an area lacking the pattern structures 0203.
Measures to inspect defects in these minute pattern structures include an optical inspection method, or so-called scatterometry, in addition to direct observation measures, such as atomic force microscopes (AFM) and scanning electron microscopes (SEM). The AFM, SEM and scatterometry are well known techniques in this technical field. In comparison with the AFM and SEM, scatterometry that utilizes an optical method can conduct faster inspections.
Scatterometry is generally a technique, as shown in FIG. 3, in which a spectral detection optical system 0301 detects a spectral reflectance 0303 of a surface of a test object 0302 to detect the shape of the repeating pattern structures 0304 uniformly formed on the surface of the test object 0302 based on the detected spectral reflectance 0303. If the uniformly formed pattern has a cross-sectionally deformed pattern structure, the structure exhibits a different spectral reflectance. Utilizing this phenomenon, the shape of the pattern uniformly formed on the surface of the test object can be detected by the spectral reflectance. The shape detection also uses some other techniques such as a model fitting approach and library matching approach.
It is known that this technique can perform the shape detection with higher sensitivity when the detected light covers a wider range in a short wavelength region.
The shape detection, by scatterometry, of the pattern uniformly formed on the test object surface can be advantageously made with light covering a wide wavelength range in a short wavelength region. However, it is not easy to realize an optical system capable of the spectral detection with light in a wide wavelength range including short wavelengths, that is, an ultraviolet range. Even if possible, such an optical system will be very expensive.
The optical system includes optical elements each having limited wavelength characteristics. Especially, among the optical elements, most half mirrors are not suitable for light having the wide wavelength range from ultraviolet to infrared. Even if there is a suitable half mirror, it will cost a lot.
JP-A No. 285761/2007 discloses one example of methods for realizing relatively inexpensive half mirrors available for light from the ultraviolet to infrared range.